U.S. patent number 4,872,755 [Application Number 07/164,790] was granted by the patent office on 1989-10-10 for interferometer for measuring optical phase differences.
This patent grant is currently assigned to Carl-Zeiss-Stiftung. Invention is credited to Michael Kuchel.
United States Patent |
4,872,755 |
Kuchel |
October 10, 1989 |
**Please see images for:
( Certificate of Correction ) ** |
Interferometer for measuring optical phase differences
Abstract
For generating several interferograms which differ from each
other in the relative phase position between the interfering
partial beams, a light source is utilized having a coherence length
less than the optical path difference between the two component
beams in the measuring path of the interferometer. Furthermore, at
least one optical delay device is provided which splits the beam
into two component beams and which generates an optical path
difference between these component beams which is approximately the
same as the optical path difference of the partial beams in the
measuring path of the interferometer. Thereafter, the delay device
again unites the component beams congruently.
Inventors: |
Kuchel; Michael (Oberkochen,
DE) |
Assignee: |
Carl-Zeiss-Stiftung
(Heidenheim/Brenz, DE)
|
Family
ID: |
6322483 |
Appl.
No.: |
07/164,790 |
Filed: |
March 7, 1988 |
Foreign Application Priority Data
Current U.S.
Class: |
356/495 |
Current CPC
Class: |
G01J
9/02 (20130101); G01B 9/02098 (20130101); G01B
9/02065 (20130101); G01B 9/02038 (20130101); G01J
9/0215 (20130101); G01J 2009/0249 (20130101); G02B
6/2861 (20130101) |
Current International
Class: |
G01B
9/02 (20060101); G01J 9/02 (20060101); G01J
9/00 (20060101); G02B 6/28 (20060101); G01B
009/02 () |
Field of
Search: |
;356/345,346,358,360,363 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"Direct Measurement of Phase in a Spherical-Wave Fizeau
Interferomenter" by R. C. Moore and F. H. Slaymaker, (Applied
Optics, vol. 19, No. 13, 7-1-1980, pp. 2196 to 2200. .
"Instantaneous Phase Measuring Interferometry" by R. Smythe and R.
Moore, (Optical Engineering, Jul.-Aug. 1984, vol. 23, No. 4, pp.
361 to 364)..
|
Primary Examiner: McGraw; Vincent P.
Assistant Examiner: Turner; S. A.
Attorney, Agent or Firm: Ottensen; Walter
Claims
What is claimed is:
1. An interferometer for measuring optical phase differences which
occur between two partial beams reflected from a reference surface
and from the surface of the part to be measured, respectively, the
interferometer comprising:
light source means for providing a coherent beam directed along a
beam path;
optical delay means for splitting said beam into first and second
component beams and for generating a first optical path difference
between said first and second component beams and for then
congruently recombining said component beams;
optical directing means for directing said first and second
component beams toward the reference surface and the measurement
surface of the part to be measured whereat said partial beams occur
and a second optical path difference is generated between said
partial beams;
said optical delay means including adjusting means for adjusting
said first optical path difference to be approximately equal to
said second optical path difference;
said light source means having a coherence length that is less than
said second optical path difference;
spatially-resolving receiver means; and,
optical means for transmitting said partial beams reflected from
said reference surface and said measurement surface, respectively,
so as to image said reference surface and said measurement surface
on said receiver means.
2. The interferometer of claim 1, said adjusting means including
fine adjust means for reproducibly changing said first optical path
difference by fractions of the wavelength of said light source
means.
3. The interferometer of claim 1, said spatially-resolving receiver
means including a plurality of spatially-resolving receivers for
simultaneously measuring a plurality of interferograms having
different phase positions; and, said interferometer further
comprising: polarization-optical means for imparting respectively
different polarizations to said component beams.
4. The interferometer of claim 1, said spatially-resolving receiver
means including a plurality of spatially-resolving receivers for
simultaneously measuring a plurality of interferograms having
different phase positions; and, said optical delay means including
at least two optical devices which are adjusted to have fixed phase
differences with respect to each other.
5. The interferometer of claim 1, said spatially-resolving receiver
means including a plurality of spatially-resolving receivers for
simultaneously measuring a plurality of interferograms having
different phase positions; said optical delay means including: at
least two optical delay devices; and, means for adjusting said
optical delay devices to fixed phase differences with respect to
each other.
6. The interferometer of claim 1, said spatially-resolving receiver
means including a plurality of spatially-resolving receivers for
simultaneously measuring a plurality of interferograms having
different phase positions; said optical delay means including: less
than three optical delay devices which are adjusted to have fixed
phase differences with respect to each other; and, said
interferometer further comprising means for generating beam paths
with additional phase differences.
7. The interferometer of claim 1, said spatially-resolving receiver
means including a plurality of spatially-resolving receivers for
simultaneously measuring a plurality of interferograms having
different phase positions; said optical delay means including: less
than three optical delay devices; and, means for adjusting said
optical delay devices to fixed phase differences with respect to
each other; and, said interferometer further comprising means for
generating beam paths with additional phase differences.
8. The interferometer of claim 1, comprising polarization-optical
means for obtaining a spatial or angular separation of the partial
beams occurring in said measuring part.
9. The interferometer of claim 1, said light source means being a
spectrum lamp.
10. The interferometer of claim 9, said spectrum lamp having a
coherence length of approximately 2 mm.
11. The interferometer of claim 1, said light source means being a
laser.
12. The interferometer of claim 1, said spectrum lamp having a
coherence length of approximately 2 mm.
13. The interferometer of claim 1, said light source means
comprising: a continuous light source; and, a narrow band
interference filter.
14. The interferometer of claim 13, said continuous light source
being a halogen lamp and said filter having a spectral half-value
width of 0.5 nm.
15. The interferometer of claim 13, said continuous light source
having a coherence length of approximately 2 mm.
16. The interferometer of claim 1, said light source being a star.
Description
FIELD OF THE INVENTION
The invention relates to an interferometer for measuring optical
phase differences which occur between two partial beams in a
measuring part. The interferometer includes a light source for
providing coherent beams and at least one spatially-resolving
receiver.
BACKGROUND OF THE INVENTION
The term "two partial beams in a measuring part" refers to the beam
path actually utilized. Further beams can occur because of the
multiple reflections. These further beams are, however, not
essential and produce at most disturbance effects. The term
"coherent radiation" is here utilized in the conventional sense for
radiation having a coherence length which is suitable for
generating interferences.
Interferometers for measuring optical phase differences are
utilized, for example, for the quantitative testing of optical
surfaces in that the test surface and the reference surface are
imaged onto a spatially-resolving receiver with an interference
pattern occurring. For each point of the interference pattern, a
sinusoidal intensity variation occurs when the reference surface is
moved in the direction of the impinging beam by a half wavelength.
These intensity curves can, for example, be stored in a computer as
a function of the movement of the reference surface and the best
possible adaptation of a sinusoidal curve is determined for every
point of the interference pattern or of the test surface. The phase
position of each individual sine curve then directly provides the
form deviation (with respect to the reference surface) of the
corresponding point of the test surface when the wavelength of the
light source used is considered.
The paper entitled "Direct Measurement of Phase in a Spherical-Wave
Fizeau Interferometer" by R. C. Moore and F. H. Slaymaker (Applied
Optics, Volume 19, No. 13, July 1, 1980, pages 2196 to 2200) shows
that it is known to use such interferometers not only for the
optical testing of plain surfaces, but also for spherical surfaces.
In this connection, the Fizeau arrangement is ever more preferred
in lieu of the conventional Twyman-Green arrangement because of the
simpler configuration. In the Fizeau arrangement, the test and
reference surfaces are not disposed in separated interferometer
arms; instead, they are disposed in the same interferometer arm
wherein they are separated from each other mostly by a wedge-shaped
air gap. With the Fizeau arrangement, the interferometer
configuration becomes considerably simpler and only the reference
surface must be produced with a high optical precision.
In the known arrangements, the reference surface must, for example,
be moved by half or a few wavelengths with high position resolution
and precisely along a straight line in order to change the phase
differences between the reference and test surfaces. For this
purpose, piezoelectric transducers are conventional. This method is
very complex for large test surfaces which require correspondingly
large reference surfaces and, from a certain size on, is no longer
realizable. Furthermore, for spherical reference surfaces, the
generated phase difference is dependent upon the aperture angle of
the corresponding beam, that is, the phase difference is not the
same for all points of the interferogram.
The paper entitled "Instantaneous Phase Measuring Interferometry"
by R. Smythe and R. Moore (Optical Engineering, July-August 1984,
Volume 23, No. 4, pages 361 to 364), discloses a Michelson
Interferometer for measuring optical phase differences. In this
interferometer, no temporal variation of the relative phase
positions between the reference wave and test wave occurs; instead,
several interferograms are measured simultaneously with several
spatially-resolving receivers. These interferograms differ in a
defined manner in the relative phase position between the reference
wave and the test wave. For this purpose, the light in both
component arms of the Michelson Interferometer is polarize
differently for "marking" the phase. The "signal decoder" utilizes
this "marking" of test wave and reference wave in order to generate
several interferograms (usually three or four) by means of further
polarization-active components. These interferograms are
distinguished one from another in a defined manner in the relative
phase position between the test wave and the reference wave.
However, this method is not applicable to a Fizeau Interferometer
because no method is known for the latter by means of which the
reference beam and the test beam can be polarized differently.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide an
arrangement which does not have the limitations discussed above and
nonetheless can influence the reference beam in a different way
from the test beam so that several interferograms are produced, one
after the other in a time sequence, or spatially one next to the
other. The interferograms differ one from the other in a defined
manner in the relative phase position between the interfering
component beams.
The interferometer according to the invention measures the optical
phase differences which occur between two partial beams in a
measuring part. The interferometer includes a light source for
providing a beam of coherent radiation and at least one
spatially-resolving receiver. According to a feature of the
interferometer according to the invention, the coherence length of
the light source is less than the optical path difference between
the two partial beams in the measuring part. Also, at least one
optical delay device is provided which splits up the beam into two
component beams and generates an optical path difference between
these component beams which is approximately equal to the optical
path difference of the partial beams in the measuring part of the
interferometer. An optical delay device brings the component beams
returning from the measuring part together again in a congruent
manner.
In a preferred embodiment of the invention, the optical path
difference of the delay device is changeable in a reproducible
manner by a fraction of a wavelength of the light source. The
optical components of the delay device can be held small
independently of the size of the reference and test surfaces so
that even for very large reference and test surfaces, the known
adjusting arrangements for a reproducible change of the optical
path difference can be applied by means of the invention.
In another preferred embodiment of the invention, several
spatially-resolving receivers are provided for making simultaneous
measurements of several interferograms having different phase
positions. For this purpose, polarizing optical means are provided
by means of which the component beams having experienced different
delays in the delay device, are polarized differently. The
invention therefore makes it possible to exploit the advantages of
a simultaneous measurement of several interferograms with fixed
phase relationships to each other without polarizing optical means
being required in the measuring part of the interferometer. In this
way, the advantages of the high insensitivity with respect to shock
and vibrations, or the measurement of fast-changing events can be
applied to numerous interferometer types.
In a further preferred embodiment of the invention, at least two
optical delay devices are provided with several spatially-resolving
receivers. The delay devices are adjusted or are adjustable to
fixed phase differences with respect to each other. Thus, even with
fewer than three optical delay devices, optical means for
generating beam paths with further phase differences are present.
Therefore, the invention makes it possible to exploit the
advantages of a simultaneous measurement of several interferograms
with fixed phase relationships to each other without requiring
polarizing optical means in the entire interferometer. In this way,
the advantages of high insensitivity with respect to shock and
vibrations or the measurement of fast-changing events can also be
applied to interferometers wherein polarization would be a
disturbance.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described with reference to the drawings
wherein:
FIG. 1 is a schematic showing a Fizeau arrangement;
FIG. 2 is an arrangement which is similar to the Fizeau arrangement
and is equipped with a plane plate;
FIG. 3 is an arrangement for testing aspherical surfaces;
FIG. 4 is a further arrangement for testing aspherical
surfaces;
FIG. 5 is an arrangement for testing a wedge-shaped plate;
FIG. 6 is a Fizeau arrangement with differently polarized component
beams and four spatially-resolving receivers;
FIG. 7 is a further Fizeau arrangement having four
spatially-resolving receivers;
FIGS. 8 and 8a is a lateral-shear interferometer having four
spatially-resolving receivers; and,
FIG. 9 is a Mach-Zehnder arrangement.
DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
The schematic of FIG. 1 shows an interferometer which corresponds
to the known Fizeau arrangement except for the delay device 10. The
light source 11 can, for example, be a semiconductor laser. The
beam emanating from light source 11 is provided with an appropriate
aperture angle by means of the lenses (12a, 12b) and the diaphragm
12c and is reflected at the beam splitter cube 12d and is again
made parallel by lens 13. The parallel beam then passes through the
planar plate 14 having the reference surface 14a from which a part
(partial beam) of the beam is reflected. The other part of the beam
passes through the air gap 15 having the optical path length
nL.sub.1 and a further part (partial beam) is reflected at the test
surface 16. The two reflected partial beams then pass through the
lens 13, the beam splitter cube 12d, a diaphragm 17a and a lens 17b
up to the spatially-resolving receiver 18. As a consequence of the
foregoing, the test surface 16 and the reference surface 14a are
imaged on the receiver 18 by means of the lenses (13 and 17b) with
an interference pattern occurring at the receiver. The interference
pattern can, for example, be evaluated in a known manner as
described in the introduction.
The delay device 10 is important for the invention and includes a
beam splitter cube 10a as well as a 90.degree.-angle mirror 10b
which can be coarsely adjusted in the directions 10r on a
mechanical guide 10m and can be finely adjusted with a piezo
element 10p. Those rays which pass over the angle mirror 10b have
an optical path difference of 2nV.sub.l relative to the remaining
rays wherein:
n is the index of refraction of air; and,
2V.sub.1 is the additional path length through the delay path.
The rays emanating from the light source 11 can reach the receiver
18 along four different optical paths OP:
OP.sub.1 without path difference 2nV.sub.1, without air gap
2nL.sub.1
OP.sub.2 with path difference 2nV.sub.1, without air gap
2nL.sub.1
OP.sub.3 without path difference 2nV.sub.1, with air gap
2nL.sub.
OP.sub.4 with path difference 2nV.sub.1, with air gap 2nL.sub.1
The 90.degree.-angle mirror is so positioned on the mechanical
guide 10m that for the difference of the optical path OP.sub.2 and
OP.sub.3, the following applies:
wherein KL is the coherence length of the radiation of the light
source 11. However, for all other possible differences of the
optical path lengths, the following applies:
The condition can be realized that the interference pattern on the
receiver 18 can only arise by means of an interference between the
component beams with optical paths OP.sub.2 and OP.sub.3 by
suitably selecting the following: the coherence length of the light
source 11, the length L.sub.1 of the air gap 15 and the length
V.sub.1 of the delay device 10. The other component beams do not
contribute to the interference pattern; instead, they simply effect
a reduction in contrast which, however, can be accepted because of
the photoelectric measurement of the irradiated intensity and the
way in which the subsequent evaluation is made. A recording of at
least three interferograms is achieved with the known method
described in the introduction with the piezo element 10p being
utilized for changing the phase differences.
FIG. 2 shows an arrangement which is similar to a Fizeau
arrangement. An advantage of the invention is that the distance
between the test surface and the reference surface does not change.
Therefore, planar plates can be tested with respect to their planar
parallelism. With the arrangement of FIG. 2, the optical phase
differences are measured between the surfaces 25a and 25b of the
planar plate 25 which for this purpose is mounted downstream of the
positive lens 13. The optical delay device 20 in this case is an
especially advantageous embodiment which includes a beam splitter
cube 20a and two retroreflecting devices (20b, 20c). The
retroreflective devices (20b, 20c) are each made up of a converging
lens 20q and a mirror 20s disposed in the focal plane of the
converging lens and have the characteristic that they reflect into
themselves the parallel rays impinging thereupon independently of
the angle of incidence. The retroreflective arrangement 20b is
coarsely adjustable on the mechanical guide 10m and the reflecting
arrangement 20c is finely adjustable with a piezo electric
transducer 10p. A planar plate 29 is mounted in the beam path ahead
of the retroreflecting device 20b, effecting a delay of 2nV.sub.2
wherein n is the index of refraction of the planar plate 29. The
planar plate 29 should be made from the same type of glass and be
of the same thickness as the planar plate 25 to be tested so that
the interference pattern on the receiver 18 can be as rich in
contrast as possible.
Maintaining these requirements for the planar plate 29 is, however,
not at all critical. It is even possible to do without the planar
plate 29, and to utilize the delay device 10 of FIG. 1, because
when the radiation of the light source 11 has, for example, a
spectral half-value width of approximately 0.1 nm and a
corresponding coherence length of approximately 4 mm, a reduced
contrast of the interference pattern is obtained without the plate
29, which nevertheless can still be tolerated. It is understood
that in this case, care must be taken that the optical path
difference between the two beam paths of the delay device (in FIG.
1 this path difference is 2nV.sub.1) is equal to twice the optical
path length of the planar plate 25, i.e. 2nL.sub.2.
In the embodiment of FIG. 2, the radiation emanating from the light
source can reach the receiver via four different optical paths. For
a suitable selection of the coherence length and the remaining
conditions, the requirements listed above with respect to FIG. 1
apply correspondingly.
FIG. 3 shows a further embodiment wherein an aspherical surface 34a
is tested against a spherical reference surface 36 and for which
the delay device 30 is not arranged in the illumination
portion.
The radiation coming from the light source 11 is collimated at lens
12a and is linearly polarized by means of the polarizer 31a in the
event that the light source does not already provide polarized
light. Thereafter, the direction of oscillation of the light is so
adjusted by means of the halfwave plate 31b that it lies at an
angle of 45.degree. to the plane of the drawing. The lens 12b
images the light source on the small diaphragm 12c. The light is
deflected by the splitter cube 12d to lens 33 which again
collimates the beam. In the delay device 30, the beam is split by
the polarization beam splitter 30a into a first component beam
polarized perpendicularly to the plane of the drawing and which is
reflected to the mirror 30b, and into a second component beam which
is polarized parallel to the plane of the drawing and which goes to
the mirror 30c. These component beams pass through separate lenses
(39b and 39a) and are subsequently brought together in a
polarization beam splitter 30d. The further configuration
corresponds completely to a Fizeau Interferometer with an air gap
35 having the optical path length nL.sub.3 between the test surface
34a and the reference surface 36.
For the returning radiation, the polarization beam splitter 30d
effects a splitting into the correct component beams within the
delay device 30. An analyzer 38 between the lens 17b and the
receiver 18 provides that the two temporally coherent waves, which
can cause the desired interference to occur, obtain the same
polarization state and therefore can interfere.
In this embodiment, the optical path difference of the delay device
30 is given by the difference of the light paths via the mirrors
(30b and 30c). This optical path difference can therefore not be
shown as a distance in FIG. 3. The polarization beam splitter 30a
and the mirror 30b are adjusted such that the optical path
difference between, on the one hand, the sum of the optical light
paths from the polarization beam splitter 30a via mirror 30b and
through the lens 39b up to the splitter surface of the polarization
beam splitter 30d, and, on the other hand, the sum of the optical
light paths from the polarization beam splitter 30a via mirror 30c
and through the lens 39a up to the splitter surface of the
polarization beam splitter 30d, correspond to the optical path
length nL.sub.3 in the air gap 35 between the surfaces (34a and
36). The additional light path after the reflection of the wave on
the surface 36 is then compensated on the return of the light in
the delay device 30 before the radiation is again united through
the polarization optical beam splitter 30a. The defined change of
the optical path difference by fractions of a wavelength is again
obtained with the piezo element 10p which moves the mirror 30c.
Also in this case, the radiation emanating from the light source
can again reach the receiver via four different optical paths which
however differ in their polarization planes in the delay device 30.
For a suitable selection of the coherence length and the remaining
conditions, the requirements set forth above for FIG. 1 are again
applicable.
The arrangement of the lenses (39a and 39b) within the delay device
30 has the great advantage that the lens 39b acts only together
with the test surface 34a, and the lens 39a acts only with the
reference surface 36 to create the interference pattern. In this
way, it is possible to test the aspherical test surface 34a with
the spherical reference surface 36. The lens 39b is so designed
that it transforms the entering planar wave into a wave which,
after passing through the beam splitter 30d and the entrance
surface 34b of the aspherical lens 34, takes on the form of this
aspherical surface at the location of the surface 34a. The lens 39a
is so designed that it transforms the entering planar wave into a
wave, which after passing through the beam splitter 30d and after
passing through the entire aspherical lens 34 and the air gap 35,
takes on the form of the spherical reference surface 36.
With the example of FIG. 3, it is shown that it can be very
advantageous in some situations if the temporally coherent waves
reflected by the test surface and the comparison surface
additionally differ in their polarization. The delay device 30
satisfies both functions in the arrangement according to FIG. 3,
namely the delay of the optical path for compensating for air gap
35 with the possibility of a defined change of the optical path
difference by a fraction of a wavelength on the one hand, and
"marking" the waves by means of their polarization on the other
hand. In FIG. 4, it will be shown that these functions can also be
separated.
A further embodiment of an optical delay device 40 is illustrated
in FIG. 4, behind the light source 11 and the collimator lens 12a.
The delay device 40 comprises a polarization optical beam splitter
cube 40e and two triple prisms (or triple mirrors) (40f and 40g).
FIG. 4 shows two interferometers arranged one behind the other,
namely, the Mach-Zehnder Interferometer made up of components (40a,
40b, 40c, 39a, 39b, 30d) and the Fizeau-Interferometer comprising
components (34 and 36). For compensating for the optical path
differences which the waves in the two interferometers encounter,
the triple prism 40g can be displaced on a guide 10m in the
direction 10r by a distance V.sub.4. The defined change of the
optical path difference by fractions of a wavelength again is
provided with the piezo element 10p which moves the triple prism
40g.
The beam emanating from the light source 11 and the lens 12a is
first linearly polarized by the polarizer 31a in the event that the
light source does not already provide polarized light. Thereafter,
the oscillation direction of the light is adjusted by means of the
halfwave plate 31b so that it lies at 45 .degree. to the plane of
the drawing. The polarization optical beam splitter 40e then
reflects the component which oscillates perpendicularly to the
plane of incidence (plane of the drawing) to the triple prism 40g,
and transmits the component which oscillates parallel to the plane
of incidence to the triple prism 40f. The recombination after the
delay of the component beam having the perpendicular oscillation
direction with the other component beam is without loss if the
triple prisms do not change the corresponding polarization
condition. It is therefore preferable to utilize triple mirrors or
to coat the reflecting surfaces of the triple prism with silver
layers.
The arrangement and function of the components (12a, 12b, 12c, 12d,
33, 17a, 17b and 18) correspond to that already described in FIGS.
1 and 2. Also here the beam splitter cube 12d is not
polarization-active. The component group comprising components
(40a, 40b, 40c, 39a, 39b, 30d) which represent a special embodiment
of a "polarization-active" Mach-Zehnder Interferometer can be
considered a special attachment which permits convex aspherical
surfaces to be compared to concave spherical surfaces in a Fizeau
arrangement downstream. It is evident that also concave aspherical
surfaces can be compared to convex spherical surfaces. A large
variety of applications are possible due to this modular
configuration. The splitting and recombining of orthogonally
polarized (and simultaneously differently delayed) waves in the
Mach-Zehnder Interferometer occurs without loss with the components
(40a and 30d) before as well as after the reflection of the waves
on these surfaces (34a and 36). The transmission axis of the
analyzer 38 is adjusted at 45.degree. with respect to the plane of
the drawing or can also be adjusted to another angle for different
reflectivities of the surfaces (34a and 36). The analyzer 38 is
mounted in front of the spatially-resolving receiver 18 and
provides that the time-coherent waves receive the same polarization
condition and can interfere.
The delay device 40 shown in FIG. 4 can be replaced by means of the
delay device 10 shown in FIG. 1 of the polarizer 31a and the
halfwave plate 31b are inserted between the lens 12a and the beam
splitter 10a and if the splitter layers in the prism 10a are
polarizing. Equally as well, the delay device 20 shown in FIG. 2
can be modified to a polarization-optically effective delay device.
For this purpose, in addition to components (31a, 31b), two further
quarterwave plates are needed which are inserted between the
splitter cube 20a and the retroreflecting units (20b, 20c). The
splitter cube 20a must then be a polarizing splitter cube. With the
aid of these embodiments, it is apparent that further arrangements
can be provided with which a simultaneous "marking" of the beams
can be obtained by means of the optical light path and the
polarization state.
A further possibility should be mentioned as to how the defined
change of the optical path difference by fractions of a wavelength
can be effected or, which is of the same significance, how the
defined change of the optical phase difference of the interfering
waves by fractions of 2pi can be effected. Until now, only
mechanical displacements of components by means of piezo elements
have been described. However, with respect to the embodiments of
FIGS. 1 or 4, it is easily possible to insert a rotating halfwave
plate into one arm of the delay device 40 with the halfwave plate
being disposed between two stationary quarterwave plates. The
optical phase difference is then changed by 4pi with a complete
rotation of the halfwave plate. In lieu of the foregoing, a
stationary and a rotating quarterwave plate can be utilized in one
arm in the delay device 20 shown in FIG. 2.
In the following, two further examples are described which show how
useful the combination of "marking" of the waves is by means of
time delay and polarization.
Similarly to FIG. 2, the first example has as its object to compare
the front and back surfaces of a massive plate with respect to each
other; however, in this instance, under the assumption that the
surfaces enclose a wedge angle.
The arrangement shown in FIG. 5 should be seen as a further
"accessory" which can be placed ahead of the lens 33 of the
arrangement of FIG. 4 in lieu of the components (40a), etc. This
accessory includes both halfwave plates (50a, 50c), the two
Wollaston prisms (50b, 50d), the afocal Kepler telescope having the
ocular 51a and the objective 51b as well as the test object 25
having the two surfaces (25a, 25b). The optically effective wedge
angle (a) of the wedge plate 25 is enlarged by the telescope to the
wedge angle a'=af.sub.2 /f.sub.1 '. With the two Wollaston prisms
and the two halfwave plates, it is possible to generate two plane
waves which enclose the angle (a') and re each linearly polarized
with oscillation directions mutually perpendicular.
It is possible to change a' continuously within certain limits
(-a'.sub.max .ltoreq.a'.ltoreq.a'.sub.max) by rotating the
components (50a, 50b and 50d). The two orthogonal polarized bundles
which enclose angle (a') enter the telescope and enclose the angle
(a) after leaving the telescope. The component having the
polarization perpendicular to the plane of the drawing of FIG. 5
(before entering the halfwave plate 50a), has already received a
delay to 2nV.sub.4 in the delay device which approximately
corresponds to the optical path 2nL.sub.5. The component
corresponding to this component after the Kepler telescope has an
oscillation direction which in the general case is no longer
perpendicular to the plane of the drawing and impinges
perpendicularly on the front surface 25a of the wedge plate 25. The
bundle polarized orthogonally hereto must pass through the
additional optical path nL.sub.5 before it impinges perpendicularly
on the back surface 25b of the wedge plate 25 and, after
reflection, again passes through the optical path nL.sub.5. With
this arrangement, the telescope made from the components (51a, 51b)
must have a good field correction which however presents no
problem.
The function of the Wollaston prisms (50b, 50d) and the two
halfwave plates (50a, 50c) is described below, first for the
adjustment for which a'=0 results and then for a finite angle
a'.noteq.0.
For a'=0, the optical axes of both halfwave plates (50a, 50c) are
adjusted parallel to the polarization directions in the incident
component. The edges of the prisms of which both Wollaston prisms
are built, are perpendicular to the plane of the drawing of FIG. 5.
In this case, the angle introduced by the first Wollaston prism is
immediately cancelled again by the second Wollaston prism. Only a
small parallel offset of the beam results which is unimportant for
the operation of the interferometer. The components (50b, 50c and
50d) are moved together as close as possible in order to hold this
parallel offset as small as possible.
The two Wollaston prisms (50b and 50d) are rotated by the same
angle in mutually opposite directions about the optical axis for
adjusting the finite angle (a'). The halfwave plate 50a is rotated
through an angle by an amount corresponding to half the angle of
rotation of the Wollaston prism 50b and in the same rotational
direction. The halfwave plate 50c remains unchanged. The first
halfwave plate rotates the polarization directions of the incident
beams parallel to the axes of the Wollaston prism 50b so that no
mixing of delayed and non-delayed waves occurs. The second halfwave
plate is stationary. By means of the second halfwave plate, the
polarization directions are adapted to the Wollaston prism 50d
(oscillation direction parallel and perpendicular to the prism
edge). In this way, a mixing of delayed and non-delayed waves is
again prevented.
The waves returning from the test object after perpendicular
reflection on the front and back surfaces pass through the
arrangement in the opposite direction and leave the latter in the
originally polarization state.
In FIG. 6, an embodiment is shown wherein no time change of the
relative phase position between comparison wave and test wave
occurs; instead, and in lieu thereof, several interferograms are
measured simultaneously with several spatially-resolving receivers.
These interferograms differ from one another in a defined manner in
the relative phase position between comparison wave and test wave.
For that purpose, the light in both component arms of the delay
device 60 are polarized differently for "marking" the phase. The
receiver device 68 utilizes this marking of the test wave and the
comparison wave to generate several interferograms (four for
example) by means of further polarization-active components. These
interferograms differ from one another in a defined manner in the
relative phase position between test wave and comparison wave. The
function and arrangement of the polarization-optical components is
disclosed in U.S. Pat. No. 4,360,271 which is incorporated herein
by reference. This patent shows a Michelson Interferometer without
spatially-resolving receivers for the purpose of making length
measurements. The application for the purpose of interferometric
testing with spatially-resolving receivers changes nothing with
respect to the modulation and decoding principle. Compared to
measuring interferograms in time sequence, the parallel measurement
affords the great advantage that the relative phase positions in
all measuring channels are changed in the same amount, for example,
as a consequence of vibrations and that therefore the phase
relationships of the measuring channels are not changed with
respect to each other. For this reason, even events which change
rapidly with time can be measured.
With the invention, the method of parallel measurement can be
applied with interferometers for which a modulation of the phase by
means of polarization optical methods in the measuring components
of the interferometer is not possible or at least is not
advantageous. A typical example is here again presented by the
Fizeau Interferometer of FIG. 1 or of FIG. 2. In the embodiment of
FIG. 1, a quarterwave plate must, for example, be inserted in the
air gap 15 between the surfaces (14a, 16) in order to polarize the
test wave and the reference wave differently. This would be wholly
unsuitable because such a large quarter waveplate cannot be
produced or at least not with the required quality. In the
embodiment of FIG. 2, the insertion of such a plate between the
surfaces (25a, 25b) is not at all possible.
In FIG. 6, the marking of the test wave and the comparison wave is
achieved via different polarization conditions in the delay device
60. The operation of the delay device 60 corresponds substantially
to that of the delay device 40 shown in FIG. 4. With respect to the
delay device 10 of FIG. 1, the polarizer 31a and the halfwave plate
31b have been added. The splitter layers of the beam splitter 60a
act here to polarize. The roof-edge mirror 10b is now no longer
mounted on the piezo element. The halfwave plate 31b is adjusted so
that the linear polarized light oscillates at 45.degree. to the
plane of the drawing. The s-component is delayed in the delay
device 60 with respect to the p-component by 2nV.sub.1. The
decoding device comprises the halfway plate 67a by means of which
the oscillation directions of the s-component and of the
p-component are both conjointly rotated by 45.degree. to the plane
of the drawing. The decoding device further includes the
non-polarizing beam splitter cube 68a which splits both components
and directs the same to the polarizing beam splitter cubes (68b,
68c). In addition, a quaterwave plate 67b is inserted between the
beam splitter 68a and the polarizing beam splitter 68b and effects
a phase delay between the s-component and the p-component by pi/2.
Finally, the polarizing beam splitters effect the production of
four interferograms for which the phase position between the
equally polarized, interfering components of the test wave and the
reference wave each differ by pi/2. These interferograms are
simultaneously measured with the synchronized spatially-resolving
receivers (18a, 18b, 18c, 18d). As in FIG. 1, the test surface 16
is sharply imaged on the spatially-resolving receivers (18a, 18b,
18c, 18d) by the lenses (13, 17b). The focal lengths of the lenses
as well as the object distance and image width must be
correspondingly selected. In FIG. 6, the proportions are not
tightly maintained so that an overview can be provided.
In the embodiments of FIGS. 1, 2, 4 and 6, the optical delay device
is disposed in the "illumination part" of the interferometer. This
affords the advantage that the possible different aberrations
arising in the component arms of the delay device can be made
substantially unharmful by means of the small pinhole diaphragm
12c. Furthermore, the cross sections of the beam can be held
especially small. An arrangement of the delay device in the
"observing part" of the interferometer however opens up an entirely
new possibility for the simultaneous generation of several phase
displaced interferograms without having to apply
polarization-optical methods for this purpose. The principle is
described with reference to FIG. 7. In FIG. 8, an especially
advantageous embodiment is illustrated.
In FIG. 7, a Fizeau Interferometer is again illustrated. The beam
splitter 68a splits the partial beams reflected from the surfaces
(14a, 16) with different delays. These partial beams are first
split into two components which are directed to the prisms (70a,
71a). The component reflected to the prism 70a is first observed.
The two partial beams contained in this component, which originate
from the reflection at the test surface 16 and the reference
surface 14a are temporally incoherent and therefore are at first
not capable of interference. The first splitter surface of the
prism 70a splits both partial beams contained in the component in a
relationship of 1:1. The half reflected at the splitter surface is
directed via angle mirror 70b to the second splitter surface of
prism 70a and is there united with the other half which in
transmission had passed through the first splitter surface of prism
70a. The two united halves now contain a portion which is
temporally coherent and therefore capable of interference since the
one half of the component beam, which was directed via the angle
mirror 70b, passed over an additional optical path 2nV.sub.11 which
corresponds approximately to the additional optical path 2nL.sub.1
in the air gap between the surfaces (14a, 16). The difference of
the optical paths between 2nV.sub.11 and 2nL.sub.1 must be less
than the coherent length of the light source. The angle mirror 70b
is displaceable along the direction 70r on the guide path 70m for
adjusting the delay V.sub.11 corresponding to the particular
distance L.sub.1. The two spatially-resolving detectors (18a, 18b)
by means of which two interferograms can be simultaneously measured
are located behind the second splitter surface of the prism 70a. In
these interferograms, the phase differences between test wave and
reference wave differ from each other at each point by pi. The
phase difference between the interfering coherent portions of the
test wave and the reference wave can be continuously and uniformly
changed for the entire interferogram by means of a fine adjustment
of the distance V.sub.11 between the prism 70a and the angle mirror
70b.
What was stated above applies in principle to the component
transmitted in the splitter cube 68a which reaches the prism 71a
and finally leads to two further interferograms which are measured
with the spatially-resolving detectors (18c, 18d). A delay
2nV.sub.12 is here adjusted which corresponds approximately to the
delay 2nV.sub.11. The phase positions of the interferograms between
the detectors (18c, 18d) again differ from each other by an amount
pi. The angle mirror 70b is finely displaced on its guide path 70m
in order to realize the desired relative phase position of pi/2
between the detectors (18a, 18c). If one assumes exactly the same
prisms (70a, 71a), the following would apply for the difference D
between the optical distances nV.sub.11 and nV.sub.12 for the
adjusted condition: ##EQU1## wherein q is a small integer number.
In this respect, the distance between the splitter cube 68a and the
prisms (70a, 71a) have no effect on the relative phase positions of
the interferograms.
As in FIG. 1, the test surface 16 should be sharply imaged on the
detectors in order to prevent diffraction fringes at the edge of
the test object. For this purpose, the focal lengths of the lenses
(13, 17b) have to be appropriately selected and the optical paths
which result from the object distance and the image distance must
be appropriately adjusted. For the purposes of providing an
overview in FIG. 7, the object distance and image distance are not
presented to scale. The transmission paths through the prisms (70a,
71a) are taken as the optical paths specifying the image distance.
The coherent portion of the partial beam reflected at the reference
surface 14a passes via the angle mirrors (70b, 71b). The image
distance of this partial beam is then too large. For different
focal lengths (13 and 17b) it is therefore not possible to image
the reference surface 14a sharply on the detector. However,
virtually no practical disadvantages result herefrom when the test
surface 16 is somewhat smaller than the reference surface 14a so
that the Fresnel diffraction fringes at the edge of the image of
the reference surface do not extend into the image of the test
surface.
In the delay devices (70, 71), two different delays of the partial
beam reflected at the reference surface 14a are provided by means
of the additional optical paths 2nV.sub.11 and 2nV.sub.12 which are
so matched that the desired relative phase positions of the four
generated interferograms of 0, pi/2, pi, 3pi/2 result. Generally,
every desired even number of interferograms can be generated
pursuant to this principle.
The arrangement of two spatially-resolving detectors on both
outputs of the second splitter surface of the prisms (70a or 71a)
affords two advantages: the available light flux is fully utilized;
and, the phase difference of pi between the two interferograms
applies exactly and must not first be produced by means of an
adjustment. The foregoing notwithstanding, it is principally also
possible to utilize only one spatially-resolving detector for each
delay unit. For this purpose, half of the light flux is not
utilized; however, it is possible to realize every desired phase
difference between the measured interferograms. Since at least
three interferograms are required for an evaluation, at least three
delay units must be provided in this case.
In FIG. 7, the interferograms on the spatially-resolving detectors
(18a, 18d) appear as mirror images compared to the interferograms
on the spatially-resolving detectors (18b, 18c). This situation is
disadvantageous if self-scanning synchronized detectors are used
for the measurement and the differences of the signals of the
detectors (18a, 18b as well as 18c, 18d) are formed immediately by
means of an analog differential amplifier. A like orientation (not
reflected) of the interferograms can be obtained when, for example,
a mirror is inserted between the prism 70a and the detector 18a as
well as between the prism 71a and the detector 18d.
As a further embodiment, a lateral-shear interferometer with the
planar parallel shearing plate 81 is illustrated in FIG. 8. This
shearing interferometer affords the advantage of providing a
relatively simple assembly as well as a simple adjustment and low
sensitivity to vibration. The shear distance is known with a high
precision and cannot inadvertently be changed if a massive parallel
plate is used as a component which effects a splitting of the wave
front to be tested into two component wave fronts sheared with
respect to each other. The shear distance is a very essential
characteristic quantity for the computed evaluation of the
interferograms which is performed later.
Shearing interferometers require no reference wave front which
remains uninfluenced by the optical system to be tested. Instead,
they derive the reference wave front from the test wave front
itself. In the lateral-shear interferometer of FIG. 8, the test
wave front is reflected on the front surface 81a and on the back
surface 81b of the shearing plate 81 and in this way the test wave
front is offset as well as being delayed in time. This time delay
is essential for the function of the invention and the lateral
offset is essential for the function of the shear interferometer.
The invention is applicable to all shear interferometers wherein
the optical paths of both sheared component waves are either
already different or can be made different. This is the case for
the vast majority of known arrangements.
With an appropriate careful adjustment of the arrangement, the
invention can also be utilized in combination with radiation
sources of unusually short coherent length such as obtained from
sunlight with a simple color filter reducing the spectral bandwidth
to 100 nm. With a centroidal wavelength of for example 500 nm, the
coherence length (KL) is then 2500 nm, that is, there is more than
an interference fringe period available for phase measurement. The
interference fringe period corresponds to an optical path
difference of 500 nm. Accordingly, the wave front generated by an
astronomical telescope can be measured during the operation thereof
with a bright star constituting the light source with a
lateral-shear interferometer which operates pursuant to the
principle with which the interferometer of FIG. 8 operates. In this
case, the entrance pupil of the telescope is imaged on the
spatially-resolving receivers (18a, 18b, 18c, 18d) and the bright
reference star is imaged in the diaphragm 17a. The measurement of
the fast-changing wave front is, for example, necessary to
compensate for the "seeing" caused by the atmosphere utilizing an
active optical component. Because of the small necessary time
constant for the control it is especially important to generate
several phase shifted interferograms which can be read out
simultaneously.
The principle described with respect to FIG. 7 is applied for the
simultaneous detection of four phase-displaced interferograms here,
however, in another embodiment. The beam collimated by the lens 17b
is now split into two parallel beams with a Kosters prism 80a. This
makes it possible to unite the two beam splitters (70a, 71a) of the
delay device of FIG. 7 into a single component 80b. The roof-edge
mirrors (70b, 71b) of FIG. 7 are now replaced by a single roof-edge
prism 80c (see FIG. 8a). One of the waves passes additionally
through the sum of the optical paths (81c, 81d) for realizing the
lateral shear in the shear plate 81 and the sum of these optical
paths (81c, 81d) corresponds to approximately the optical path in
the roof-edge prism 80c. It is then preferable to select the
optical path in the roof-edge prism 80c to be somewhat smaller than
the additional optical path in the shearing plate so that the delay
of the delay device can be adjusted by displacing the roof-edge
prism 80c on the guide 10m along the direction 10r. The shear plate
and the roof-edge prism are advantageously made of the same glass
material. A total of four phase shifted interferograms are
generated with the delay device 80 described here. With this delay
device 80, it is essential that the interferograms on the detectors
(18a, 18c) are phase-shifted with respect to each other by a fixed
phase angle which is preferably pi/2. The same applies then for the
interferograms on the detectors (18b, 18d). In order to reach this
objective, a thin layer is vapor-deposited on the one half of the
base side of the roof edge prism (the partition line extends
perpendicularly to the 90.degree.-edge of the prism). This thin
layer then defines an additional optical path of quarter wave for
the beam which passes through. The quarter wave corresponds to an
eighth wave when the beam enters and another eighth wave when it
exits the prism. With the index of refraction being (n) for the
layer, then the following equation applies for the layer thickness:
d=.lambda./[8(n-1) ]. The index of refraction (n) is then so
selected that it is possible to subsequently vapor-deposit
reflection-reducing layers onto the entire base side of the
roof-edge prism. The simplest circumstances are obtained when the
index of refraction of the layer and of the prism material differ
from each other as little as possible.
In several applications, it is possible to start with a fixed
predetermined delay of the waves in the measuring part of the
interferometer. For reasons of the stability of the adjustment, it
is then preferable to tightly glue the prism 80c to the beam
splitter 80b. In this case, the optical path difference of
.lambda./4 can be obtained for both halves of the prism in that,
for example, an appropriate layer is vapor-deposit onto half of one
of the short faces.
A still further embodiment of the invention is shown in FIG. 9
wherein the invention is applied to a Mach-Zehnder Interferometer.
This type of interferometer is often used for investigating
boundary layers, flow and convection processes, temperature
distributions and the like in transparent gases or liquids. For
this purpose, large beam cross sections or a large "test volume" 95
is needed. In the known way of applying the phase measuring
technology, the optical path length (and therefore the phase) for
example of the reference arm of the interferometer is changed in a
defined manner in that, for example, the large mirror 91b is
displaced mechanically or piezo-electrically by a fraction of a
wavelength. However, this application fails here because of the
size of the mirror.
In FIG. 9, the delay device 90 is mounted between the light source
11 and the pinhole diaphragm 12c. The coherence length of the light
source is again less than the optical light path 2nL.sub.9 which is
additionally passed through by the second component wave in the
reference arm of the Mach-Zehnder Interferometer. The interference
capability between the component waves is again established by
means of the optical delay device 90. The optical delay device 90
here comprises the Kosters prism 90a, the two triple prisms (90f,
90g) as well as the surface mirrors (90c, 90d) which are vapor
deposited onto the exit surface of the Kosters prism. In this delay
device, tilting as well as lateral displacement of the triple
prisms have no harmful effect. The triple prism 90g is adjustable
on a guide path 10m in the direction 10r for adjusting the
coherence. The triple prism 90f is mounted on a piezo element 10p
for providing defined changes of the phase position of the
component waves. The triple prism 90g is displaced on the guide
path 10m in the direction 10r by an amount V.sub.9 .apprxeq.L.sub.9
/2 for compensating for the additional optical path 2nL.sub.9 of
the second component wave in the Mach-Zehnder Interferometer. A
precondition for the foregoing is that the medium in the test
volume 95 has approximately an index of refraction of n=1. If this
is not the case, compensation can be achieved by inserting a
corresponding "reference volume" into the reference arm of the
Mach-Zehnder Interferometer between the mirrors (91b and 91d). The
reference volume is then filled with a medium having the same index
of refraction.
The beams which are time delayed differently are again united by
the Kosters prism 90a and expanded with the lenses (12b, 93a). The
small diaphragm 12c lying between the lenses (12b, 93a) serves to
clean the beams. The beam now enters the Mach-Zehnder
Interferometer consisting of the two beam splitters (91a, 91c) and
the two mirrors (91b, 91d). The test volume 95 is imaged on the
spatially-resolving detector 18 on which the interferences arise,
by means of the lenses (93b, 17b). The diaphragm 17a eliminates
possible disturbing interferences which can originate on the
rearward side of the beam splitter plates which are slightly
wedge-shaped.
It is understood that the foregoing description is that of the
preferred embodiments of the invention and that various changes and
modifications may be made thereto without departing from the spirit
and scope of the invention as defined in the appended claims.
* * * * *